0

I need some subfloor advice, please.

1. What size screws do you use to screw down the 3/4” plywood Subfloor to the floor joists?

2. The kitchen has two layers installed already. 3/4” Subfloor with a 5/8” underlayment. But to level it with the living room floor, I need to install an additional layer of 5/8” plywood. What length of screws should I use to secure the new 5/8” plywood to the old 5/8” plywood?

3. The bedroom already has a 3/4” Subfloor installed. To level it with the rest of the house, I need to install an additional layer of 3/4” plywood. What length of screws should I use to secure the new 3/4” plywood to the old 3/4” plywood?

4. Lastly. When installing these layers of underlayment. Do I need to screw into the floor joists or just the other layer of underlayment?enter image description here

I appreciate any help and advice with this. Thank you.

1
  • Do any of the floors squeak at all? This would or could effect the length of screws needed.THe age of the house might help too.
    – Jack
    Jun 1, 2023 at 2:25

1 Answer 1

1

There are some real experts here, so if they come with better answers then I'll probably delete this. But...

You always want to go into the joists. The only catch is that there could be stuff (water pipes, electrical wiring) going through the joists. Normal practice is to have at least 1-1/4" clearance. To play it safe, I would aim for 1" of screw into the joist and add that to the thickness of plywood, underlayment, etc. For your situation, that would be approximately:

  • Kitchen: 5/8 + 5/8 + 3/4 + 1 = 3"
  • Bathroom: 3/4 + 1 = 1.75"
  • Pantry, Bedroom Closet: 3/4 + 3/4 + 1 = 2.5"

If a few screws don't hit the joists, you generally don't have to remove them. They won't hurt, they just won't hold the floor together as securely as the screws that are into the joists.

5
  • Thank you very much for writing me back about this. The house is getting all new plumbing and electrical installed. Thankfully, no electrical or plumbing will go through any flooring before I put in the subfloor and underlayments. Do I need to add any glue to the plywood or only just to the subfloor layers? May 30, 2023 at 3:27
  • Glue? I don't know. My one experience with adding subfloors in my kitchen really didn't go so well - 23 years later (and really for several years now) and it is a big mess coming apart...maybe glue would have helped! May 30, 2023 at 3:30
  • Just wondering if your joists were not quite level with each other. Screws/nails should be plenty to hold a floor/sub floor in place, if there is no force to being flat(dips/ridges).
    – crip659
    May 30, 2023 at 10:25
  • 2
    @crip659 assuming the subfloor material itself is flat!
    – Huesmann
    May 30, 2023 at 12:39
  • 1
    Glue and screw is best practice for squeak-free floors. May 30, 2023 at 15:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.